The principles of the silver complex diffusion transfer reversal process, hereinafter called DTR-process, have been described e.g. in U.S. Pat. No. 2,352,014 and in the book "Photographic Silver Halide Diffusion Processes" by Andre Rott and Edith Weyde--The Focal Press--London and New York, (1972).
In the DTR-process non-developed silver halide of an information-wise exposed photographic silver halide emulsion layer material is transformed with a so-called silver halide solvent into soluble silver complex compounds which are allowed to diffuse into an image receiving layer and are reduced therein with a developing agent, generally in the presence of physical development nuclei, to form a silver image having reversed image density values ("DTR-image") with respect to the black silver image obtained in the exposed areas of the photographic material.
A DTR-image bearing material can be used as a planographic printing plate wherein the DTR-silver image areas form the water-repellant ink-receptive areas on a water-receptive ink-repellant background. The DTR-image can be formed in the image receiving layer of a sheet or web material which is a separate element with respect to the photographic silver halide emulsion material (a so-called two-sheet DTR element) or in the image receiving layer of a so-called single-support-element, also called mono-sheet element, which contains at least one photographic silver halide emulsion layer integral with an image receiving layer in waterpermeable relationship therewith. It is the latter mono-sheet version which is preferred for the preparation of offset printing plates by the DTR method.
Two types of the mono-sheet DTR offset printing plate exist. According to a first type disclosed in e.g. U.S. Pat. No. 4,722,535 and GB-1,241,661 a support is provided in the order given with a silver halide emulsion layer and a layer containing physical development nuclei serving as the image-receiving layer. After information-wise exposure and development the imaged element is used as a printing plate without the removal of the emulsion layer. This type of printing plates has a low printing endurance, typically around 10000 copies.
According to a second type of mono-sheet DTR offset printing plate a hydrophilic surface of a support, mostly anodized aluminum, is provided in the order given with a layer of physical development nuclei and a silver halide emulsion layer. After information-wise exposure and development the imaged element is treated to remove the emulsion layer so that a support carrying a silver image is left wich is used as a printing plate. This type of printing plates has a much higher printing endurance, typically at least 25000 copies. Such type of lithographic printing plates is disclosed e.g. in U.S. Pat. No. 3,511,656, EP-A-278766, EP-A-410500 and EP-A-483415.
The process by which the printing plate is obtained should be an easy, ecological and ergonomical one. Yet, removing the emulsion layer e.g. by treating the developed imaging element with an aqueous solution, what is the most usual procedure can be a costly and cumbersome process, requiring the use of hot water and a finisher comprising large amounts of trypsin.
The use of hot water has several disadvantages. The cost of hot water is high. Moreover, hot water dissolves the proteinic binder, usually gelatin, of the emulsion layer, thus causing decomposition of said layer so that a dirty black waste water comprising silver particles and dissolved silver salts is obtained, which upon cooling may clog filters and draining pipes. Moreover, trypsin is extraordinarily expensive and is ecologically harmful.
According to U.S. Pat. No. 5,068,165 these disadvantages have been circumvented for the major part by providing between the aluminum foil and the silver halide emulsion layer a thin water-swellable intermediate layer comprising for at least 70% of its total weight at least one non-proteinic hydrophilic film-forming polymer.
However, as a consequence, the deposition of silver on the aluminum surface and the sharpness of the transferred silver image are reduced.
According to U.S. Pat. No. 5,273,858 these disadvantages have been circumvented by providing between the aluminum foil and the silver halide emulsion an intermediate layer comprising hydrophobic polymer beads prepared by polymerization of at least one ethylenically unsaturated monomer and having an average diameter not lower than 0.2 .mu.m. However, said solution is still not completely satisfactory.